为什么在刷新页面的时候也会触发Button_Click事件呢？

初次发文，请多关照！

源程序如下：
using System;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Web;
using System.Web.SessionState;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.HtmlControls;
using System.Web.Util;
using System.Web.Mail;

namespace SendEMail
{
 public class WebForm1 : System.Web.UI.Page
 {
 protected System.Web.UI.WebControls.TextBox txtAttach;
 protected System.Web.UI.WebControls.TextBox txtContent;
 protected System.Web.UI.WebControls.Button btnsend;
 protected System.Web.UI.WebControls.Button btnClose;
 protected System.Web.UI.WebControls.TextBox txtSender;
 protected System.Web.UI.WebControls.TextBox txtReceiver;
 protected System.Web.UI.WebControls.TextBox txtTitle;
 protected System.Web.UI.WebControls.TextBox txtCC;

 private void Page_Load(object sender, System.EventArgs e)
 {
 }

 override protected void OnInit(EventArgs e)
 {
  InitializeComponent();
  base.OnInit(e);
 }

 private void InitializeComponent()
 {   
  this.btnsend.Click += new System.EventHandler(this.btnsend_Click);
  this.Load += new System.EventHandler(this.Page_Load);

 }

 private void btnsend_Click(object sender, EventArgs e)
 {
  MailMessage mail = new MailMessage();

  mail.From = txtSender.Text;
  mail.To = txtReceiver.Text;
  mail.Cc = txtCC.Text;
  mail.Subject = txtTitle.Text;
  //mail.Attachments.Add(new MailAttachment(txtAttach.Text));
  mail.Attachments.Add(new MailAttachment("D:\\input.txt"));
  mail.BodyFormat = MailFormat.Text;
  mail.Body = txtContent.Text;
  //mail.BodyEncoding = MailEncoding.Base64;
  mail.Priority = MailPriority.High;
  SmtpMail.SmtpServer = "172.18.100.200";
  SmtpMail.Send(mail);
 }
 }
}

我想问的问题是：
   为什么刷新页面的时候会进入到btnsend_Click这个时间里面去？